In a typical radio receiver, an antenna receives radio frequency signals and converts them into electrical radio frequency signals. These radio frequency signals are then reduced to a lower frequency signal for amplification in an intermediate frequency (IF) stage. The intermediate frequency stage can be seen in FIG. 1. In the intermediate frequency stage the signal received from the antenna is amplified and bandpass filtered. Then the signal is split and input to a quadrature demodulator. The demodulator reduces the frequency of the signal to the base band frequency and prepares the signals for extraction of the useful information. The base band circuitry filters, amplifies and processes the signal extracting the analog or digital data.
A first source of distortion in the base band frequency signal is common to a majority of such radio receivers. The distortion is the result of extraneous noise coupled into the receiver circuitry causing inaccuracies in the resulting data. Some of the extraneous noise is the result of the local oscillators being coupled into the amplifier stages with an unknown phase shift. This results in an undesired DC component in the resulting demodulated base band frequency signal.
A second source of distortion in the base band frequency signal occurs when there is a change in the received signal strength. This can occur when a receiver changes from one frequency to another or when the source of the received signal changes. The relationship between the two signal strengths results in a change of the gain requirements of the automatic gain controller AGC 103 located at the beginning of the intermediate frequency stage shown in FIG. 1. This change in gain changes the resulting DC component of the base band frequency signal.
Other sources of distortion in the base band frequency signal are a result of imperfect amplifying stages and demodulator circuitry.
The results of these added undesired DC components to signals used for digital data transmission are illustrated in FIG. 3 and FIG. 4. FIG. 3 shows an ideal representation of an arbitrary signal set, represented in signal space by the Xs 301 projected onto the inphase (I) and quadrature (Q) axes 303. In FIG. 4, the results of adding an undesired DC offset 309 to either the I or the Q component is illustrated. The shifts in the I and Q components cause the decision threshold to shift, thus, biasing the selection of one symbol over the other, reducing the noise margin for some of the symbols and allowing a reduced margin for error in the presence of uncorrelated noise.
The undesired DC offset error introduced from the local oscillator is often acceptable for some radio receivers, however, digital radio receivers have a lower tolerance to offset errors than a conventional analog radio receiver. The undesired DC offset introduced from changing signal strengths is usually never a problem in a single frequency radio receiver, however, in tunable radio receivers it can be a problem. The problem is transitory in nature and occurs upon tuning the radio receiver to a new frequency. This may be acceptable for some applications and many conventional radios use resistor (R) capacitor (C) networks and others have added a switch to change the impedance of the RC network during a transition to reduce the amount of time for the transition to settle. The straightforward solution of an RC network to remove the undesired DC component, better known as AC coupling, becomes biased when a long sequence of symbols, representing patterns of 1s and 0s, received by a digital receiver do not have an equal number of 1s and 0s, therefore, causing a desired DC component which would be eliminated by AC coupling the signal. There exists a need for a device to correct for the undesired DC offset quickly prior to receiving the data of interest on the radio receiver. Such a device should correct for transient errors that occur when tuning to a new frequency and would quickly adapt to the correct voltage level. There exists a need to correct in an extremely short time for the undesired DC offset caused from changing the receiver gain and coupling of the local oscillator in the IF stage.